This invention relates to a method and device for determining USDA Bostwick consistency values of a non-Newtonian fluid, and more particularly, for determining the consistency of a thick material such as tomato paste, directly from a material production stream without conducting presently complicated required USDA procedures for determining USDA Bostwick consistency values.
Consistometers such as the Bostwick consistometer developed by E. P. Bostwick of the USDA in 1938, and given official status for use in consistency grading of pureed products such as tomato catsup were developed to establish a fixed set of values for grading these products according to the subjective quality known as consistency. Another consistometer developed for measuring the consistency of such purees is the modified Adams consistometer. Typically, both the Bostwick and the Adams consistometer are very simple devices which measure the amount of slump or flattening out of a puree over a predetermined time interval. The derivation of mathematical equations describing Bostwick consistometer performance is difficult, yet it has found wide application in the food industry because of its ease of operation and good correlation with consistency as the consumer subjectively senses this property. However, these devices are only good for batch measurements and highly inadequate in continuous processes.
It has been proposed as an alternative to employ one of many well built and mathematically calculable viscosimeters to determine consistency as a method of adaptation to continuous processes. However, laboratory tests have shown that viscosimeters do not measure the same properties as does the Bostwick device, and in fact, the Bostwick device does a much better job in indicating actual consumer preferances.
The problem with using a viscosimeter to measure consistency is that each measurements treat a puree as if it is a true liquid, with the viscosity being measured at a single selected shear rate. This approach requires two assumptions. First, that one deals with a true normal liquid and, second, that the first assumption is probably in error but that there is sufficient proportionality between the measured and real properties to make the measurements of value. Since the proportionality is not constant, and in some cases varies greatly, the resulting measurements can be highly misleading.
Another attempt to solve the problem of consistency measurements of continuous flows is discussed in the article entitled, The Plastometer--A New Development in Continuous Recording and Controlling Consistometers by David Eolkin, Food Technology Journal, Jan. 11, 1952. This article discusses a device which branches-off a partial flow from a product line into a dual bridge and causes pressure drop differences to arise by varying respective tube diameters. These measurements are then employed to measure the structure of the component product. On the other hand, as discussed in page 256 of the article, the true consistometer does not distinguish between the consistency and viscosity but measures the system as a whole. Thus, in some instances the resulting measurements from the plastometer will be quite different from actual consistency values. Further, in no way does this device attempt to correlate measurements with USDA Bostwick.